This invention relates to a method for treating electroplating wastewater. In a more specific aspect, this invention concerns itself with a method involving the use of ferrous sulfate and sodium sulfide to effect the simultaneous reduction of hexavalent chromium contaminants and precipitation of heavy metal contaminants from electroplating wastewater.
In general, electroplating wastewaters bearing heavy metals such as copper, cadmium, nickel, and chromium, are relatively easy to treat in chemical precipitation systems. The effluent quality obtained is limited only by the solubility of the metal salts formed in the reaction. This is because these heavy metals all readily form hydroxides or sulfides, with the notable exception of chromium; which will not form a sulfide at standard temperature and pressure. Additionally, chromium generally requires an additional treatment step to reduce the ion from the hexavalent to the trivalent state. Many treatment chemicals can be used for this reduction. Among these are ferrous sulfate, sodium bisulfite, sulfur dioxide, and sodium sulfide. While all these chemicals work well from the standpoint of effluent quality, the quantity of sludge produced by the different processes can vary dramatically.
With the advent of the Resource Conservation and Recovery Act (RCRA), producing clean water is no longer sufficient. Hauling and disposal charges for hazardous sludge are over $100/ton in many areas, hence the volume of sludge for disposal is nearly as important as the effluent quality. Some of the more exotic treatment chemicals, such a sodium borohydride, are extremely efficient from a sludge production standpoint (8 moles of electrons are available per mole of reactant), but they are quite expensive and have not been fully tested on mixed metal wastewaters. To address this problem, a concerted research effort was undertaken to investigate the sludge volumes produced by the more common reduction chemicals, and to provide a more efficient and economical treatment procedure. Even though wastewater treatment plants must treat mixed-metal wastes, chromium was singled out for this investigation because it alone requires both reduction and hydroxide precipitation.
Present treatment technology for chromium reduction is predicated on the fact that the rate of chromium reduction depends upon the pH of the waste solution. Taking the "standard" practice as ferrous reduction the reaction is: EQU 3Fe.sup.2+ +HCrO.sub.4 --+7H.sup.+ .fwdarw.3Fe.sup.3+ +Cr.sup.3+ +fH.sub.2 O (1)
The rate of this reduction reaction was quantified as: ##EQU1## Since this rate equation is third order with respect to the hydrogen ion concentration ([H.sup.+ ]), it has been the basis for claims that chromium reduction is very slow at all but acidic pH levels. Each unit increase in pH (i.e., decrease in [H+]) would decrease the rate of the reaction by three orders of magnitude. For example, it has been calculated that reducing 100 milligram per liter (ppm) of hexavalent chromium at pH 3 would take over 1000 times longer than at pH 2 (90 minutes vs. 5 seconds).
Completely contrary to the rate question, as shown in equation (2) above, others have indicated that hexavalent chromium could be rapidly reduced at pH 8.0. Additional follow-up work in this technical area, however, has clarified this apparent dichotomy. The work illustrated by equation (2) was done with very large (and, hence, assumed constant) concentrations of ferric ion (Fe.sup.3+) at a very acidic pH. Under these acidic conditions, this assumed constant ferric concentration was valid. However, at near neutral or at alkaline pH levels, the solubility of the ferric ion depends upon the concentration of hydroxide (OH.sup.-) as follows: EQU Fe.sup.3+ +OH.sup.- .fwdarw.Fe(OH).sub.3 ( 3)
Using the solubility product, EQU K.sub.sp =[Fe.sup.3+ ][OH.sup.- ], (4)
and the disassociation constant for water, EQU K.sub.w =[H.sup.+ ][OH.sup.- ], (5)
and combining and substituting them into equation (2), the result is an expression independent of pH. ##EQU2##
This research work has opened the full pH range for chromium reduction reactions. In light of this, the pH conditions for effecting a chemical reduction was investigated with respect to sludge production in the research effort which culminated with the present invention.